1. Field of the Invention
The present invention relates to a door drive mechanism of an air conditioning system for a vehicle, and particularly to the door drive mechanism of the air conditioning system, which drives a door, such as an air mix door, through a gear mechanism.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 2000-355212, which corresponds to U.S. Pat. No. 6,354,935, discloses a door drive device, which drives a door, such as an air mix door, of an air conditioning system (also referred to as a vehicle air conditioning system) for a vehicle.
The above door drive device includes a door drive gear, which is rotated by a drive apparatus, such as a motor. The door drive gear is meshed with a rack, which is integrated with the air mix door. When the door drive gear is rotated by the drive apparatus, the air mix door is slid together with the rack.
FIG. 5 is an air mix door drive gear mechanism of a previously proposed air conditioning system, which is different from that of the above Japanese Unexamined Patent Publication No. 2000-355212. In the air mix door drive gear mechanism, a driving side gear 30r is secured to an output shaft 29r of a drive servo motor (not shown) and rotates integrally with the output shaft 29r. A driven side gear 31r, which is meshed with the driving side gear 30r, is secured to a rotatable shaft 19r of an air mix door (not shown) and rotates integrally with the rotatable shaft 19r. 
The driving side gear 30r is formed into a circular shape having a constant pitch circle radius (i.e., a constant radius of a pitch circle). Corresponding to this constant pitch circle radius of the driving side gear 30r, a pitch circle radius of the driven side gear 31r is also constant. An operable angular range of the air mix door is less than 360 degrees, and thus the driven side gear 31r has a fan shape, which has an arcuate outer peripheral part. The arcuate outer peripheral part of the driven side gear 31r extends along an imaginary arc that is centered in the rotational axis of rotatable shaft 19r. 
As discussed above, both the pitch circle radius of the driving side gear 30r and the pitch circle radius of the driven side gear 31r are constant, so that a linear relationship exists between an operational angle of the driven side gear 31r and an operational angle of the driving side gear 30r, as indicated by a characteristic A (a characteristic line indicating a relationship between an operational angle of the driving side gear and an operational angle of the driven side gear) in FIG. 6.
This will cause the following disadvantages in terms of control of the temperature (hereinafter referred to as an outlet air temperature) of air to be discharged from the vehicle air conditioning system into the passenger compartment. In a maximum heating position of the air mix door, the air mix door fully closes a cool air passage (an air passage that bypasses a heating heat exchanger) and fully opens a warm air passage (an air passage that extends through the heating heat exchanger) in a unit case. When the air mix door is rotated from the maximum heating position to a temperature control range where both the cool air passage and the warm air passage are opened, the air mix door opens the cool air passage. Since this cool air passage extends along the entire extent of the unit case in a width direction of the unit case (a left-right direction of a vehicle), this movement of the air mix door results in a rapid increase in a flow rate of cool air, which flows through the cool air passage.
Thus, when the air mix door is driven from the maximum heating position to the temperature control range, the outlet air temperature of the air conditioning system is rapidly reduced, as indicated by a characteristic C (a characteristic line indicating a relationship between an operational angle of the driving side gear and the outlet air temperature of the air conditioning system) in FIG. 7, resulting in poor controllability of the outlet air temperature.
To address such a disadvantage, it is conceivable to add an auxiliary link mechanism between the driven side gear 31r and the rotatable shaft 19r of the air mix door to reduce the amount of change in the operational angle of the driven side gear near the maximum heating position of the air mix door to reduce the amount of change in the rotational position of the air mix door, as indicated by a characteristic B (a characteristic line indicating a relationship between an operational angle of the driving side gear and an operational angle of the driven side gear) of FIG. 6. However, this requires the addition of the auxiliary link mechanism besides the gears 30r, 31r. Thus, an additional space for accommodating the auxiliary link mechanism needs to be provided, and the addition of the auxiliary link mechanism causes an increase in manufacturing costs.
In the above described air conditioning system recited in Japanese Unexamined Patent Publication No. 2000-355212, a tooth depth of the tooth located at the end of the door drive gear or a tooth depth of each of the teeth located near the end of the door drive gear is increased relative to the rest of the teeth. Also, a tooth depth of the tooth located at each of the ends of the rack or a tooth depth of the teeth located near each of the ends of the rack is increased relative to the rest of the teeth. However, this structure is intended to move the air mix door in a direction perpendicular to a slide direction of the air mix door at the end of a slide path of the air mix door, and there is no structure of the gear mechanism disclosed to improve the controllability of the outlet air temperature.